This is an application in response to RFA-NS-17-00 4 ?BRAIN Initiative: Optimization of Transformative Technologies for Large Scale Recording and Modulation in the Nervous System (U01)?. In a proof-of-concept publication we demonstrated a novel fiber-coupled miniature microscope that achieved 3D confocal imaging of fluorescently labeled neurons in brain tissue using electrowetting lens technology for axial focusing. This low weight instrument (<2 g) is suitable for surgical implantation to achieve all optical awake behaving recording with axial focusing deep in the mouse brain, a significant advance compared to commercially available devices. In this proposal we present a major breakthrough demonstrating successful imaging through the fiber coupled microscope using two-photon microscopy. Leveraging on this result we propose to optimize our proof of concept E-FCM (electrowetting-fiber coupled microscope) by making it compatible for two-photon excitation (2PE-FCM) suitable for recording and modulating neural activity in awake behaving mice. Our 2PE-FCM will incorporate holographic two-photon opsin stimulation with a spatial light modulator (SLM) for rapid 3D volumetric two-photon imaging and modulation of neuronal activity. Optimization of the 2PE-FCM will be achieved by regular interactions of beta users with a multidisciplinary group of physicists, engineers and neuroscientists in a parallel process of design implementation and testing the device. Testing of the 2PE-FCM will employ a step-by-step approach designed for effective generation of a device suitable for studying local circuits in different brain areas in different species. We propose a solid dissemination plan partnering with Intelligent Imaging Innovations Inc. to commercialize the device. Validation of the 2PE-FCM will be a major advance in all-optical interrogation of neural circuits creating a new venue for understanding the neural basis of behavior. Aim 1. TECHNICAL APPROACH: Optogenetic activation and simultaneous readout of deep brain neurons using a novel fiber coupled microscope with a variable focus electrowetting lens using no moving parts. Aim 2. MILESTONES AND TIMELINE: Validate and disseminate the 2PE-FCM device for studies of the neural basis of behavior in freely behaving animals.